The present invention relates to a process for preparing fluoropyridines from chloropyridines by treatment with hydrogen fluoride.
Fluoropyridines, such as 3,5-dichloro-2,4,6-trifluoropyridine, are valuable intermediates useful in the preparation of herbicides, insecticides, fungicides, fixed dyes, and other end use products. Certain of these compounds are taught to be preparable from the corresponding chloropyridines by exchange with alkali metal fluorides, either in a melt and in certain dipolar, aprotic solvents. Certain of them are also taught to be preparable from the corresponding chloropyridines by exchange with hydrogen fluoride, either in the vapor phase or neat. None of the methods are entirely satisfactory, however. Methods employing an alkali metal fluoride produce an alkali metal chloride as a by-product which must be disposed of, sold, or recycled. Known methods employing hydrogen fluoride (see, for example, British Patent Application No. 1,272,475, published Apr. 26, 1972) are slow, require metal halide or carbon catalysts, and produce only low yields of the desired fluoropyridines.